In the prior art, in order to supply to an IC (Integrated Circuit) a voltage obtained by rectifying an alternating-current voltage or a direct-current voltage, a resistive voltage divider element has been used to voltage-divide the voltage obtained by rectifying the alternating-current voltage supplied from a high-voltage line or the direct-current voltage (see for example Patent References 1 to 3 below). Here, a high-voltage line is a line which supplies high voltages of 100 V or higher.
FIG. 34 is a circuit diagram showing principal portions of a voltage divider circuit of the prior art. The voltage divider circuit 2200 of the prior art comprises a resistor 2221 and resistor 2222. The output of the voltage divider circuit 2200 is input to the IC 2230. The resistor 2221 is connected between a high-voltage line 2210 and an input terminal of the IC 2230. One end of the resistor 2222 is connected to the resistor 2221, and the other end of the resistor 2222 is grounded.
The resistor 2221 and resistor 2222 form a resistive voltage divider element which performs resistive dividing of a voltage obtained by rectifying an alternating-current voltage or of a direct-current voltage. The resistor 2221 and resistor 2222 lower the voltage obtained by rectifying an alternating-current voltage or direct-current voltage to a low voltage which can be input to the IC 2230, and input the voltage to the IC 2230. The input terminal of the IC 2230 is connected to the intermediate node between the resistor 2221 and the resistor 2222.
FIG. 35 is a circuit diagram showing a switching power supply of the prior art. The switching power supply of the prior art shown in FIG. 35 is a power factor improvement circuit 1800. The power factor improvement circuit 1800 performs full-wave rectification of the AC input of a commercial power supply at, for example, 100 to 240 V using a first rectifier 1801 comprising a diode bridge, and uses this voltage to charge a power supply capacitor 1802. Then, a switching transistor 1804 is controlled by a power factor improvement control IC 1803, a step-up inductor 1805 passes current intermittently, and the intermittent current is converted into a sine wave by a second rectifier 1806 and a first capacitor 1807 and is output.
An IN terminal 1816 is provided in the power factor improvement control IC 1803. The IN terminal 1816 is connected to the intermediate node of a first resistive voltage divider circuit 1809, comprising two resistors 1809a and 1809b and connected in parallel with the power supply capacitor 1802. This is in order to use the first resistive voltage divider circuit 1809 to resistively voltage-divide the voltage obtained by using the first rectifier 1801 and power supply capacitor 1802 to rectify the alternating-current voltage, and to input the result to the IN terminal 1816.
And, based on the signal input from the IN terminal 1816, the power factor improvement control IC 1803 outputs a pulse width control signal to the gate terminal of the switching transistor 1804.
A resistor 1810 is connected between the power factor improvement control IC 1803 and the switching transistor 1804; by means of this resistor 1810, the gate voltage of the switching transistor 1804 is adjusted. Further, a resistor 1811 lowers the voltage output from the first rectifier 1801 to a desired power supply voltage, and supplies the result to the power factor improvement control IC 1803. In this way, the resistors 1809a, 1809b, 1810, 1811 are mounted externally to the power factor improvement control IC 1803.
Here, specific operation of the control portion 130 is explained. When the voltage of a ZCD terminal 1813 falls, a set signals is input from COMP 1814 to RSFF 1815, and the switching transistor 1804 is turned on. The voltage-divided voltage of the IN terminal 1816 and VREF 1817 are compared by the AMP 1818, and this signal and the triangle wave signal generated by RAMP 1819 are compared by COMP 1820; if the output signal from AMP 1818 is lower than the RAMP 1819 signal, a reset signal is input to RSFF 1815 from COMP 1820, a low signal is output from OUT 1821, and the switching transistor 1804 is turned off. Further, if the voltage at the IS terminal 1822 exceeds VOCP 1823, a reset signal is input to RSFF 1815 from COMP 1824, a low signal is output from OUT 1821, and the switching transistor 1804 is turned off.
FIG. 36 is a circuit diagram showing a modified example of a switching power supply of the prior art. In the power factor improvement circuit 1800 shown in FIG. 36, the same symbols are assigned as in the similar configuration of the power factor improvement circuit 1800 shown in FIG. 35, and explanations are omitted. Here, in addition to the first resistive voltage divider circuit 1809, a second resistive voltage divider circuit 1808 is provided on the outside of the power factor improvement control IC 1803.
Specifically, a MUL terminal 1827 of the power factor improvement control IC 1803 is connected to the intermediate node of the second resistive voltage divider circuit 1808, comprising a resistor 1808a and a resistor 1808b. This is in order that the voltage resulting from rectification of an alternating-current voltage by the first rectifier 1801 and power supply capacitor 1802 can be resistively voltage-divided by the second resistive voltage divider circuit 1808, the high voltage lowered to a low voltage that can be input to the power factor improvement control IC 1803, and this voltage input to the MUL terminal 1827.
FIG. 37 is a circuit diagram showing a modified example of principle portions of the voltage divider circuit of the prior art shown in FIG. 34. In the voltage divider circuit 2200 shown in FIG. 34, a startup element 2240 is comprised. The startup element 2240 is connected between the high-voltage line 2210 and the input terminal of the startup circuit 2250 (see for example Patent Reference 4 below).
Patent Reference 1: Japanese Patent Application Laid-open No. H11-150234
Patent Reference 2: Japanese Patent Application Laid-open No. 2005-94835
Patent Reference 3: Japanese Patent Application Laid-open No. 2007-123926
Patent Reference 4: Japanese Patent Application Laid-open No. 2008-153636
However, in the technology of the prior art of FIG. 34, the resistor 2221 and resistor 2222 are connected in series between the high-voltage line 2210 and ground, so that even during standby of the IC 2230, direct current continues to flow from the high-voltage input portion of the high-voltage line 2210 to ground via the resistor 2221 and the resistor 2222. Hence power is consumed by the voltage divider circuit 2200.
Similarly, in the technology of the prior art of FIG. 35, the resistor 1809a and the resistor 1809b are connected in series, so that even during standby of the power factor improvement control IC 1803, direct current continues to flow via the resistor 1809a and the resistor 1809b. Hence power is consumed by the first resistive voltage divider circuit 1809. Further, because the first resistive voltage divider circuit 1809 is mounted externally to the power factor improvement control IC 1803, the number of externally mounted components increases, and the cost of the semiconductor device rises.